In the disclosure of the present invention reference is mostly made to the treatment of diabetes by delivery of insulin or other diabetes drugs, however, this is only an exemplary use of the present invention.
Drug delivery devices in the form of injection devices for subcutaneous administration of fluid drugs have greatly improved the lives of patients who must self-administer drugs and biological agents. Drug injection devices may take many forms, including simple disposable devices that are little more than an ampoule with an injection means or they may be highly sophisticated electronically controlled instruments with numerous functions. Regardless of their form, they have proven to be great aids in assisting patients to self-administer injectable drugs and biological agents. They also greatly assist care givers in administering injectable medicines to those incapable of performing self-injections.
In particular pen-style injection devices have proven to provide an accurate, convenient, and often discrete, way to administer drugs and biological agents, such as insulin. Typically, injection devices use a pre-filled cartridge containing the medication of interest, e.g. 1.5 or 3.0 ml of insulin or growth hormone. The cartridge is typically in the form of a generally cylindrical transparent ampoule with a needle pierceable septum at one end and an opposed piston designed to be moved by the dosing mechanism of the injection device. The injection devices are generally of two types: “Durable” devices and “disposable” devices. A durable device is designed to allow a user to replace one cartridge with another cartridge, typically a new cartridge in place of an empty cartridge. In contrast, a disposable device is provided with an integrated cartridge which cannot be replaced by the user; when the cartridge is empty the entire device is discarded.
A further distinction can be made for the drive means delivering the force to move the cartridge piston forwards during expelling of a dose of drug. Traditionally injection devices have been manually actuated by the user pushing an extendable button during expelling, however, alternatively the driving force may be provided by a spring being pre-strained or strained during dose setting and subsequently released, this allowing for “automatic” dispensing of drug.
Despite their advantages such automatic drug delivery devices “compete” on size with the more traditional devices for multiple uses without any auto dosing feature. Therefore there is a desire to provide smaller automatic drug delivery devices which may be realized by new designs of the expelling mechanisms.
Correspondingly, drug delivery devices have been proposed that aims at reducing the size by utilising a compression drive spring or the like to deliver the drug automatically. However, the compression drive spring force and an activation force responsible for activating delivery of the drug often both will be axially working forces acting on a common element from opposite directions, e.g. for pen or pen-like drug delivery devices having an end-mounted release button. In such devices, a compression drive spring will even provide an axial force that increases linearly with the set dose. Therefore, a user wishing to activate the drug delivery must work against and even overcome this relatively high force, and even higher with higher doses, in order to activate the drug delivery.
As an example, WO 2011/060786 discloses an “autopen” driven by a compression spring strained during dose setting, the pen comprising a drive nut in threaded engagement with a piston rod, the drive nut being rotationally locked but axially moveable relative to a drive tube. The compression drive spring is arranged inside the drive tube and acting between the drive nut and the proximal end of the drive tube. When a set dose is released the drive tube is moved distally against the force of the spring and into engagement with the drive member to thereby rotate the piston rod.
Having regard to the above, it is an object of the present invention to provide an automatic drug delivery device that reduces the axial force needed to release an expelling mechanism comprising a compression drive spring. The device should be reliable in use and should be provided in a cost-effective manner. Certain designs for such drug delivery devices involve the use of a rotating drive tube for automatic drug delivery which complicates matters further. Correspondingly, it is a further object of the present invention to reduce the force needed to release an expelling mechanism of the above type using a rotating drive tube.